The invention relates to optical conductors, more specifically a light conductor made of a transparent material such as optical glass or transparent plastics, such as methacrylate or polystyrene.
It is known that light may be conducted through rods of glass or other transparent materials. Heretofore, such conductor rods have been manufactured for practical purposes mostly by combining a plurality of so called optical fibers into bundles of larger diameter. The fibers in such a bundle are arranged in such a manner that the position of any individual fiber within the bundle is the same at the beginning and at the end of the respective light conductor. Such light conductors permit the transmission of images whereby the individual image points are transmitted separately in each of the fibers making up the bundle.
The light transmission by means of a rod or of a fiber or a bundle of fibers is possible due to a total reflection. A light ray entering the facing end of such a rod is reflected in zig-zag fashion at the outer surfaces of the rod which face each other. The reflection is repeated until the light ray exits at the opposite end of the rod or bundle. In order to achieve an efficient light transmission, it is rather important that the total reflection is accomplished even for rather small total reflection angles a.sub.g. This critical angle of total reflection is important because those light rays the angle of incidence of which is below said critical angle would not be reflected back into the rod or conductor, rather such light beams would penetrate the outer surface of the rod and would thus be lost for the transmission. This critical angle may be calculated from the so called Snellius equation as follows: EQU sin a.sub.g = n'/n
In the foregoing equation n is the refraction index of the material of which the rod is made and n' is the refraction index of the medium surrounding the conductor rod. In order to achieve an efficient light transmission it is necessary that the surrounding medium is optically thinner than the conducting material of the light conductor. In other words, the surrounding medium must have a smaller refraction index than the material of the conductor rod because otherwise there will be no total reflection of the light beams.
For example, if the rod is made of a material having a refraction index of 1.5 and if the conductor rod is surrounded by air having a refraction index of about 1.0, the respective critical angle a.sub.g would correspond to about 42.degree. according to the above equation. However, if the rod of the same material is surrounded by another material having a refraction index of 1.4, the critical angle a.sub.g will increase to about 70.degree.. This means that the angle of incidence of the light onto the outer surfaces of the conductor may be the steeper the larger the difference between the two contiguous media while still assuring the total reflection. This fact becomes even more important when the rod includes curved sections or where the rod or bundle of optical fibers is flexible and has bends therein. The angle of incidence will be steeper in the curved sections of the conductor due to geometrically considerations. Thus, if the bend in the conductor becomes too pronounced, it is possible that light losses may occur. This means, that the light conductor having a large difference between the refraction indices of its two media, may be bend to a larger extent than a conductor where the respective difference between the refraction indices is smaller. Thus, the conductor having the larger difference between the refraction indices of its materials will have smaller light losses.
Incidentally, the total reflection of a light conductor disappears completely at those points at which the outer surfaces of the light conductor are in contact with a medium having a larger refraction index or where the light conductor is in contact with an opaque medium.
As a result of the above considerations, certain problems have arisen in the actual manufacture of light conductors and in the use of such conductors. Since such a glass rod or an optical fiber cannot freely float in space merely surrounded by air, if it is desired to use it as a light conductor, it is necessary that the light conductor is supported by another optical medium which is optically thinner. Such contact with another optical medium cannot be avoided, for example, due to the fact that a plurality of optical fibers are bunched together or that the conductor is arranged in a protective envelope or that it is supported at certain points along its length. The refraction indices or materials so far known to be suitable for light conductors and for envelopes of such conductors are within the range of about 1.4 to 1.6. Accordingly, only small differences between the respective refraction indices may be accomplished.
Further problems result due to the fact that it is desirable to make the light conductors flexible. Where glass is used the flexiblity can be achieved only by reducing the diameter of the individual optical fibers which reudction has its limitations, because the thinner the fiber, the larger is the risk that it will break. Heretofore, the glass fibers have been coated in a rather complicated procedure by a glass having a smaller refraction index than the glass of the fiber itself. The so coated fibers are then bunched and cemented to each other whereupon the entire bundle is provided with a protective envelope. All these steps constitute a rather expensive production process whereby the range of applications of such light conductors is rather limited to certain uses which justify the high production costs.
In view of the above, experiments have been made to produce light conductors from synthetic materials as mentioned above. However, even the use of synthetic materials has not been without problems because the suitable synthetic materials have indices of refraction which are substantially equal to each other. On the other hand, such synthetic materials have quite satisfactory optical characteristics and they also have the necessary flexiblity. Nevertheless, it is difficult to achieve a substantial difference between the index of refraction of the light conductor proper and the respective index of the coating. Attempts to fill a hose of synthetic material having a low index of refraction with a liquid having a higher index of refraction have not been quite satisfactory because even in this type of arrangement the difference between the refraction indices remains rather small. Besides, it is not possible to expose such liquids to larger temperature differentials. However, for practical reasons, it is not possible to avoid exposing light conductors to such temperature differentials. Another disadvantage results from the need that the ends of the light conductor must be closed by plugs or the like, whereby substantial sealing problems are created and additional refraction surfaces are produced. Thus, other attempts have been made to arrange between the light conductor core and a protective envelope a layer of air. The required spacing or distance between the conductor core and the protective envelope should then be limited to spacer means providing a point or line contact between the core and the envelope. A line contact is accomplished, for example, where a triangular light conductor core is inserted into a circular protective envelope. In such an embodiment, the contact between the triangular core and the envelope is limited to the corners or edges of the triangle. However, even such solutions to the problem are not quite satisfactory because each point of contact between the light conductor and the protective envelope completely prevents the total reflection at such point and the resulting light losses accumulate along the length of the conductor.